The present invention relates to a coating liquid for forming a low-permittivity silica film, which is capable of forming an insulating film that has a low relative permittivity and has excellent adherence to a substrate surface, has excellent mechanical properties, has excellent chemical resistance such as alkali resistance and also crack resistance, and which is capable of smoothing the irregularities of a substrate surface to a satisfactorily high degree. Further, the present invention relates to a substrate comprising this low-permittivity silica coating film.
Interlayer insulating films are disposed between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer and between metal wiring layers in order to effect an insulation therebetween in a semiconductor device having multilayer wiring in accordance with an enhanced integration of semiconductor device.
When, for example, a metal wiring layer is disposed on a semiconductor substrate, irregularities occur on the semiconductor substrate by the metal wiring layer. Further disposing a metal wiring layer or the like on the surface with the irregularities may cause wire breakage attributed to the level differences of the irregularities. When the level differences of the irregularities are greater than the range of exposure focal depth of resist for forming fine patterns, fine patterns cannot be obtained. Therefore, it is requisite that surfaces having irregularities attributed to the above interlayer insulating films disposed between a semiconductor substrate and a metal wiring layer, and between metal wiring layers and attributed to various elements, be smoothed to a satisfactorily high degree.
With respect to the structure having the above multilayer wiring, in a semiconductor device of 0.3.mu. rule or less, the metal wiring spacing is so small that the metal wiring impedance due to electrostatic induction is increased, resulting in a possible delay of response speed and an increase of electric consumption. Therefore, it is requisite that the relative permittivity of the interlayer insulating films disposed between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer or between metal wiring layers be minimized as much as possible.
The interlayer insulating film used for the above purposes is generally formed on a substrate by the vapor phase growing method such as the plasma CVD method or sputtering method, or the coating method in which an insulating film is formed with the use of a coating liquid for film formation.
However, in the vapor phase growing method such as the plasma CVD method, it is believed that the limit of the relative permittivity of obtained coating film is 3.5 attained by the fluoride doped silica film, and forming a coating film having a relative permittivity of up to 3 is difficult. Although the relative permittivity of the coating film formed with the use of a coating liquid comprising a fluoride doped polyimide resin or a fluororesin is about 2, the coating film has drawbacks in that, for example, the adherences to a substrate surface and to resist material used in fine working are poor and the resistances to chemical and to oxygen plasma are poor.
The conventional coating liquid for forming silica film, comprising a partial hydrolyzate of alkoxysilane, enables obtaining a coating film with a relative permittivity of 2.5 but has a drawback in that the adherence to a substrate surface is poor.
The present invention has been made with a view toward solving the above drawbacks of the prior art. It is a primary object of the present invention to provide a coating liquid for forming a low-permittivity silica film, which is capable of forming an insulating film that has a relative permittivity as low as up to 3 and is excellent in adherence to a substrate surface, has excellent mechanical properties, chemical has resistance such as alkali resistance, and also crack resistance and which is capable of smoothing the irregularities of a substrate surface to a satisfactorily high degree. It is also an object of the present invention to provide a substrate comprising this low-permittivity silica coating film.